We have previously demonstrated that retroviruses have been transferred, under natural conditions, between vertebrate species that are only remotely related phylogenetically. Some of these transfers have resulted in the subsequent incorporation of the retroviral genome into the germ cell DNA. Five examples of retroviral transfer have been documented, including a transfer from Old World primates to domestic cat ancestors, from New World primates to skunk ancestors, and from rodents to the ancestors of the domestic cat, pig, and mink. There are also two examples of virus transfer between sympatric species that have not resulted in incorporation of the acquired genome into the germ line. One case involves the transfer of endogenous viruses of Southeast Asian rodents to gibbons and the other is the transfer of endogenous retroviruses of langurs to macaques. The direction of retroviral transfer between species can be established by examining the cellular DNA of related species for viral sequences. In the New World primate (squirrel monkey) to skunk transfer, no sequences related to the squirrel monkey retrovirus (SMRV) were detected in mink and weasel cellular DNA. These two species are believed to be close relatives of the skunk. A phylogenetic tree of the carnivores was, therefore, derived from the thermal stability measurements of nonrepetitive cellular DNA; it differs in several respects from the classical relationships derived by anatomical considerations and the fossil record. The two species of skunks examined are accorded separate family status and shown not to be closely related to the Mustelids (minks and weasels). The lentiviruses isolated from goats (CAEV), sheep (visna), horses (EIAV) and man and primates (HTLV-III/LAV, STLV-III, MnIV) are present only in infected individuals of those species. The origin of these viruses is currently being examined; the identification of the endogenous host species might permit control of the vectors for virus spread.